(1) Field of the invention
This invention relates to a method of operation for vapor phase polymerization of olefins. More particularly, the invention relates to a method for starting the operation so as to reduce the formation of sheet-like polymer in the initial stage of the process of polymerization or copolymerization of .alpha.-olefins in a vapor phase fluidized bed.
(2) Description of Prior Art
When the polymerization of a-olefins is carried out in a vapor phase fluidized bed, the formation of sheet-like polymer is liable to occur in the initial stage of the polymerization and the sheet-like polymer blocks up the outlet for polymer product and other parts such as pipings in the downstream side. The blocking sometimes makes the operation substantially impossible to be continued.
The formation of sheet-like polymer is liable to occur during the period from the feeding of a catalyst into a reactor before the polymerization to the stage in which a certain quantity of polymer is produced. In other words, the sheet-like polymer is seldom formed in the regular reaction stage after the passage of the initial stage.
Accordingly, it is considered that the formation of sheet-like polymer is caused by the differences in some conditions in the period between the initial starting stage from the conditions in the regular reaction state of the polymerization system.
In the initial stage of polymerization, it is liable to occur that the sheet-like polymer is formed as well as that the bulk density of the polymer obtained in the initial stage is lower than that of the product obtained in the stable regular operation.
In the polymerization using a vapor phase fluidized bed, one of the factors to have influences on productivity is the bulk density of polymer produced. Because the productivity is determined by the weight of polymer produced per unit time with a certain volume of a reactor, when the bulk density of the polymer is increased, the productivity can be improved. Accordingly, it is desirable to maintain constantly a higher bulk density from the initial stage to and through the regular or steady state period of the polymerization.
Furthermore, polymer products are taken out from a reactor intermittently and the volume of polymer to be taken out during each time is predetermined. In the case that the bulk density of a polymer is low, the quantity of polymer particles in the discharged gases is small, so that the volume of the entrained gases discharged together with the polymer of a certain quantity is increased. The entrained gases consist of unreacted gases including nitrogen and ethylene. It is not advantageous in view of economy to recover the ethylene by separating it from the unreacted gases. Therefore, if it is possible to make the bulk density of polymer product in the initial stage as large as the value of the normal product obtained in the regular operation period, the quantity of the above-mentioned entrained unreacted gases can be reduced and the process can be improved in view of economy and production efficiency.
As described above, the productivity and economy can be improved by avoiding the lowering of the bulk density of polymer produced in the initial stage of polymerization.
Furthermore, even when hydrogen gas is fed in a predetermined gas ratio in the initial stage of polymerization, the melt flow rate (MFR) of polymer is often different from the MFR value of the polymer obtained in the regular operation period. In other words, it is observed that the function of hydrogen to control the molecular weight is not normal.
When such a phenomenon is caused to occur, trial and error operation must be repeated by changing the gas composition, measuring the changed values in MFR of obtained polymers and feeding a resultant value back to the gas composition. In the vapor phase fluidized bed operation, however, it takes many hours to replace all the polymer with a new polymer in a reactor because the residence time of the polymer particles is generally as long as several hours.
Accordingly, if the value of MFR can be maintained at a normal value from the initial stage of polymerization, the regular state production can be started producing neither wide specification product nor second-grade product.
It is disclosed in U.S. Pat. No. 5,077,358 that the seed polymer is treated with an organoaluminum compound prior to the polymerization of olefins in order to cause the compound to react with wager contained in the seed polymer (ibid., column 8, lines 32-39). In this patent, it is proposed that the seed polymer is placed in a low moisture condition, for example, by bringing it to contact with nitrogen before the treatment of the seed polymer with the organoaluminum compound. The reason for this is such that, when the seed polymer is brought into contact with an environment of a high moisture content and the seed polymer is then treated with the organoaluminum compound, the agglomeration of seed polymer is caused to occur (ibid., column 8, lines 45-54). In other words, according to this patent, the object to treat the seed polymer with an organoaluminum compound is to avoid the formation of agglomerates in a polymerization process. It is also described that the agglomerated lumps of polymer in polymerization is formed by the agglomeration of seed polymer particles.
In addition to the agglomerated lumps formed in polymerization due to the agglomeration of seed polymer, it is considered that the agglomerated lumps of polymer including those of prepolymer are formed also by the agglomeration of solid catalyst component that is fed into the reaction system, the formation of which is not restricted to the initial period of polymerization.
The sheet-like polymer referred to in the present invention is generated such that the polyolefin is adhered to the inner wall of the polymerization reactor and fused to form a polyolefin sheet and it is peeled off from the wall surface, which is literally in the form of a sheet. Even though the reason is not yet obvious, the formation of sheet is liable to occur in the initial period of polymerization. On the other hand, it is observed that the sheet-like polymer is hardly formed in the stable or steady-state operation period (regular operation).
Although the cause of the formation of polymer sheet has never been sufficiently elucidated, the mechanism of the sheet formation is hypothesized by the present inventors as follows:
When the polymerization is made to proceed, a part of the catalyst adheres on the inside wall surface of the reactor. Because the portion near the adhered catalyst hardly becomes turbulent, the generated heat of polymerization is accumulated there and formed polymer is melted to form the sheet. When the formed sheet grows to have a certain thickness, it is released from the inside wall surface of a reactor and it is mingled into the reaction system, which causes the disarrangement of the fluidized state and the blocking of pipings. The catalyst referred to above includes not only the solid catalyst component but also polyolefin particles having polymerization activity.
Because the formation of sheet-like polymer is observed in the initial period of polymerization, the tendency for the adhesion of catalyst to the inside wall of the polymerization reactor, in other words, the likelihood of the catalyst to adhere, depends upon the conditions in the reaction system in the initial period of polymerization including the pretreatment of the reaction system and it is not influenced by the conditions of polymerization after that.
The important point to avoid the formation of sheet-like polymer is not the seed polymer or produced polyolefin particles themselves but the catalyst which is adhered to the inside wall of polymerization reactor. Supposing that seed polymer particles or formed polyolefin particles (having no polymerization activity) are adhered to the inside wall, if no catalyst is adhered, there occurs neither polymerization reaction nor the generation of heat of polymerization. The sheet of fused polymer is formed by the accumulation of the heat of polymerization, if the thickness of adhered catalyst or a mixture of catalyst and polymer particles is relatively small, the heat is diffused without difficulty and the sheet-like polymer is not formed. Therefore, the sheet-like polymer is formed only when the thickness of the adhered catalyst is larger than a certain value.
In other words, it is believed that the formation of sheet-like polymer largely relates to the adhesion of catalyst to the inside wall of the polymerization reactor and the degree of such adhesion. The force to adhere the catalyst is produced by Coulomb forces of static electricity in the electrically charged catalyst component and polymer particles. Accordingly, the condition of the adhesion relates to the charged state of the solid catalyst component and polymer particles in the reaction system.
The inventors have found out that the condition of the adhesion of catalyst to the inside wall of a polymerization reactor can be controlled by treating not only the seed polymer but also the whole reaction system with an organoaluminum compound. In practice, the treatment is done after the first feeding of seed polymer particles into a reactor but prior to the start of polymerization. In other words, the tendency of catalyst to adhere to the reactor wall can be regulated by this treatment.
The organoaluminum compound reacts with water and many other impurities and loses its function. The impurities exist not only in the mass of seed polymer but also in the dead spaces in the reaction system such as the portions near the inside walls of the reactor and the pipings. Furthermore, the impurities exist on and near the inside walls of the reactor and pipings in a chemically or physically adsorbed state. Therefore, the removal of impurities is quite difficult. For example, the removal of impurities is difficult by the so-called purging with an inert gas, olefin gas, hydrogen, or a mixture of them. Even when the removal of impurities is possible, the time length necessary for the purging is very long, so that such a measure cannot be adopted in practical working. What is worse, because the impurities are not always volatile, it is not possible to remove all impurities by the purging.
When the quantity of organoaluminum compound used is large to excess in the above treatment, the ratio of low molecular weight polymer increases in the product of the initial period of polymerization, which undesirably raises the value of MFR of the obtained polymer. In this case, until the excess organoaluminum compound is completely consumed after the start of polymerization, the polyolefin having a higher MFR value is produced, which is uneconomical. In the use for film formation, such a product is not only uneconomical but also undesirable because the polyolefin used for producing films desirably contains a small quantity of lower molecular weight polymer in order to avoid blocking of prepared films.
However, because the kinds and quantities of impurities in a reaction system to consume the organoaluminum compound cannot be measured beforehand, it is necessary to estimate the quantity of the organoaluminum compound required for treating the reaction system by some measures.
Incidentally, in the foregoing U.S. Pat. No. 5,077,358, it is described that, when excess organoaluminum compound is used to treat seed polymer, hot spots are brought forth by the increased polymerization activity and agglomerates of polymer particles are liable to occur (ibid., column 9, line 63 to page 10, line 3). When the seed polymer is not treated with an organoaluminum compound, the agglomerates of polymer particles are also caused to occur. Accordingly, when the quantity of organoaluminum compound is more than a certain amount or less than a certain amount, the formation of agglomerates of organoaluminum compound may be caused to occur in both of the cases.
However, in accordance with the experiments carried out by the present inventors, the formation of sheet-like polymer is not influenced substantially by the excess feed of organoaluminum compound and the excess feed of the organoaluminum compound is rather preferable for the purpose of avoiding the formation of sheet-like polymer. It is understood, however, that a polymer having a high MFR value is produced as described in the foregoing passage. Accordingly, it is considered that the cause and conditions for of the formation of sheet-like polymer referred to in the present invention is different from those for the formation of the agglomerates described in the above-mentioned U.S. Patent.